Method of reviewing detected defects

ABSTRACT

A method to solve the problem of a technique generally used to detect a defect of a semiconductor by calculating the differential image based on pattern matching, which requires that a reference image must be picked up to pick up an image of the inspection position in an area with the semiconductor pattern having no periodicity, resulting in a low throughput. The image of the inspection position is divided into local areas, each local area is matched with the local area of the image already stored and the difference between the local areas thus matched is determined to extract area.

BACKGROUND OF THE INVENTION

The present invention relates to a method of inspecting the externalappearance and reviewing defects of a semiconductor, or in particular toa technique for increasing the speed of an apparatus for reviewingdefects of a semiconductor.

The conventional method of inspecting and reviewing defects of asemiconductor generally employs a comparative inspection in which theimage of an inspected portion is compared with the image of thecorresponding portion of a conforming article, and the differencebetween them is extracted as a defect. The method in which the images ofthe external appearance of the portions of the same design of differentchips in the same wafer are compared for inspection is called a chipcomparison method, while the method in which the images of the externalappearance of the portions designed to have the same external appearancein the same chip are compared for inspection utilizing thecharacteristics of the areas having a periodic pattern such as a memorycell is called a cell comparison method. In the inspection areas adaptedfor the cell comparison, the semiconductor pattern can be considered tohave a predictable periodic pattern. The first category of the cellcomparison method is disclosed in JP-A-2000-67243, in which a singleperiodic pattern is stored in advance as a reference image and comparedwith a plurality of images of a plurality of inspected portions therebyto extract a defective area. In the second category of the cellcomparison method, as disclosed in JP-A-2000-195458, a periodic patternis divided into a plurality of rectangular areas, each of which isdisplaced by an integer multiple of the pattern period, and it isdetermined that a defect exists in a rectangular area associated withthe maximum total sum of the differences.

The first category of the cell comparison method presupposes that theperiodic pattern is identical at each position of the wafer. It istherefore difficult to successfully meet the situation where a pluralityof different periodic patterns exist in a wafer. Also, with regard tothe first category of the cell comparison method, in spite of thedescription about the mode for automatically determining whether thepattern at an inspection position is periodic or not, it is difficult todetermine automatically only from an image whether a given pattern isperiodic or not. Further, the process for automatic determination of aperiodic pattern is accompanied by a great amount of calculations, andthe first category of the method requires a waiting time before thisdetermination process is completed. Furthermore, the second category ofthe method, which presupposes that the pattern at the inspectionposition is periodic, is not applicable in the case where the patternperiodicity is unknown.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus forreviewing defects of a semiconductor which is capable of operating athigh speed with a high throughput.

Another object of the invention is to provide a method of reviewingdefects in which a reference position constituting a normal portion canbe prevented from being contaminated by an electron beam in the casewhere the image pick-up means is a SEM (Scanning Electron Microscope).

Specifically, according to this invention, there is provided a method ofreviewing defects, comprising:

-   -   the defect image pick-up step for moving the field of view of        the microscope to a sample inspection position and picking up an        image of the external appearance of the inspection position;    -   the reference image pick-up determination step for determining        whether the image of the external appearance of a reference        position designed to have the same external appearance as the        inspection position is to be picked up or not;    -   the reference image pick-up step for moving the field of view of        the microscope to the reference position and picking up an image        of the external appearance of the reference position in        accordance with the result of determination in the reference        image pick-up determination step;    -   the defect area extraction step for extracting a defect area of        the inspection position from selected one of the image of the        external appearance alone of the inspection position and both        the image of the external appearance of the inspection position        and the image of the external appearance of the reference        position in accordance with the result of determination in the        reference image pick-up determination step; and    -   the defect area post-extraction step for is executing a process        based on the result of extraction of the defect area;    -   wherein the reference image pick-up determination step includes        at least one of the image pick-up pre-start reference image        pick-up determination step for provisionally determining, before        the defect image pick-up step, whether to pick up the image of        the external appearance of the reference position and the image        pick-up post-start reference image pick-up determination step        for finally determining, after the defect image pick-up step,        whether to pick up the image of the external appearance of the        reference position; and    -   wherein the defect image pick-up step or the reference image        pick-up step is executed concurrently with the image pick-up        post-start reference image pick-up determination step, for a        different inspection position than the inspection position        determined in the image pick-up post-start reference image        pick-up determination step.

These and other objects, features and advantages of the invention willbe apparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view showing a general configuration of a reviewsystem according to the invention.

FIGS. 2A to 2D are sequence charts for explaining the flow of imagecomparison according to the invention.

FIG. 3 is a timing chart for the sequence of operation according to theinvention.

FIG. 4 is an image picked up from a wiring pattern.

FIG. 5 is a diagram showing an image picked up from a wiring pattern andrectangular areas.

FIGS. 6A to 6C are diagrams showing an inspected image and a storedimage.

FIG. 7 is a diagram showing an inspected image.

FIG. 8 is a block diagram showing a general configuration of a systemfor carrying out the review sequence using the design data according tothe invention.

DESCRIPTION OF THE EMBODIMENTS

An embodiment of the invention will be explained with reference to FIGS.1 to 8.

FIG. 1 is a diagram showing a general configuration of a review systemfor analyzing a defect generated in a semiconductor wafer according toan embodiment of the invention. Reference numeral 101 designates asample to be inspected which is mounted on an XY stage 110. Numeral 102designates an electron gun for radiating electrons on the sample 101through electron scanning units 103. An image pick-up unit 104 issecondary electron detection means. Numerals 105, 106 designatenarrow-angle electron detector for detecting electrons from a givennarrow angle. The narrow-angle electron detectors 105, 106 are arrangedin different directions for detecting electrons at narrow angles. Theelectron scanning units 103 are controlled by a control unit 107 fortwo-dimensionally scanning the electrons emitted from the electron gun102.

Numeral 108 designates an image memory for storing the outputs of theelectron detectors 104, 105, 106. Electrons are two-dimensionallyscanned and radiated by the electron scanning units 103, and thereforean image having a different characteristic for each detector is storedin the image memory 108. In image processing unit 111, the images storedin the image memory 108 are processed and a defect is extracted. Animage of the defect thus extracted is displayed on a display screen. Thenarrow-angle electron detectors 105, 106 can produce an output which isstronger, the smaller the angle between the direction of each detectorand the normal to the sample 101. By use of this characteristic, forexample, the slope of the sample can be detected. Thus, athree-dimensional shape can be estimated, for example, as disclosed inJP-A-60-249008.

Numeral 109 designates sequence controller capable of moving theinspection view of field by controlling the XY stage. In collaborationwith the controller 107, the sequence controller 109 controls the entiresequence of the operation of the system for reviewing a plurality ofinspection points in a semiconductor defect. A defect extraction methodusing this configuration is disclosed, for example, in Japanese PatentApplication No. 2001-217510 filed earlier by the present inventors, inwhich an image of an inspection position is compared with an image of areference position corresponding to the inspection position thereby toextract a defect.

The patterns that the sample is to have at the inspection position andthe reference position are required to coincide with each other. Asdescribed in JP-A-2000-67243, therefore, a technique for extracting adefect utilizing the periodicity of the pattern of the inspectionpositions is generally known, in which the periodic pattern imaged inadvance is compared with the image picked up at each inspectionposition, by switching between the mode requiring no process of pickingup an image of a reference position for each inspection position and themode for picking an image of a reference position representing acorresponding coordinate position in an adjoining chip for eachinspection position. This technique has the function of automaticallydetermining whether a repetitive pattern is involved in the case whereit is not determined in advance whether the image of a referenceposition is not required to be picked up. This function, however, isaccompanied by the problem of a reduced throughput and the difficulty ofstable automatic determination. Further, due to the inapplicability toother than a repetitive pattern, the technique cannot be used for thesystem LSI or the like which has recently come to be mass-manufactured.The reduced throughput will be explained first.

A semiconductor review system generally requires that the stage is movedto the defect position detected by an automatic wafer inspection deviceand, after more accurately determining the defect position at a lowmagnification, the image of the defect position is picked up at a highmagnification. In an application of the method described inJP-A-2000-67243, however, as shown in FIG. 2 thereof, it is required todetermine whether a repetitive pattern is involved or not based on theimage of the inspection position, and unless a repetitive pattern is notinvolved, to move the stage to the reference position and extract adefect position by picking up an image thereof. After this, the stage ismoved to the defect position again and an image of the defect positionis picked up with the defect located at the center thereof.

In the mode of picking up an image of a reference position for eachinspection position, the image is picked up at the reference positionwith a low magnification, and then at the inspection position with a lowmagnification. After detecting a defect position using the two images oflow magnification, an image at and around the detected defect positionis picked up with a high magnification by controlling the electronscanning units 103. In the mode of automatically determining whether arepetitive pattern is involved or not, the stage is required to be movedone more time than otherwise, and also the processing time forautomatically determining whether a repetitive pattern is involved ornot is required. In the case where it is found that a repetitive patternis not involved, therefore, the time cost is so high that the throughputcannot be improved.

This invention proposes a method of solving this problem. According tothis invention, in order to eliminate the need of moving the field ofview to the inspection position again after moving it to the referenceposition in the case where a repetitive pattern is not found to beinvolved, an image of the inspection position is picked up with a largefield of view at high magnification, which is accompanied by an frameintegration. Generally, in the case where an image is picked up usingthe electron microscope, the frame integration is carried out in orderto improve the S/N ratio. In the frame integration, electrons arescanned on the inspection field of view a plurality of times, and theelectrons detected by each scan are added up. The frame integration, ifcarried out a great number of times, requires a long image pick-up time,especially for a large field of view and a high magnification.Generally, therefore, it is actually disadvantageous to extract adefective area by comparing images with a large field of view at highmagnification. The image pick-up time can be shortened by increasing thenumber of times the frame integration is carried out for picking up animage of the inspection position while reducing the number of times theframe integration is carried out for picking up an image of thereference position.

The S/N ratio of the image picked up at the reference position where theframe integration is carried out a fewer number of times is very low.Nevertheless, an image equivalent to a low-magnification image having agreater number of frame integrations can be obtained by subjecting theimage and a low-pass filter to the convolution operation with downsampling. Comparison of images with different numbers of frameintegrations shows different effects of the charging on the image. Thus,an image having the same number of frame integrations as at thereference position is required at the inspection position. Therefore,two images are generated at the inspection position, one associated witha time point during the frame integration process when the number offrames integrated becomes equal to that integrated for the image pickedup at the reference position, and the other associated with a time pointafter the frame integrations are carried out the number of timesrequired to obtain the final image quality. The effects of this methodwill be explained on the assumption that the image pick-up time perframe is 20 ms, the number of frame integrations is 12, the stage ismoved for one second, the ratio of high to low magnification is 2.5, thetime required for gain adjustment and auto focusing before picking up animage is 300 ms, the time required for automatic determination of arepetitive pattern is 500 ms and the time required for defect positionextraction is 800 ms.

Flowcharts used for comparison are shown in FIGS. 2A to 2D. In FIG. 2A,numeral 201 represents a more strict version of the flow description inJP-A-2000-67243. The time required for picking up a defect image at lowmagnification is 540 ms including the pre-processing, and the subsequentprocess for automatic determination of a repetitive pattern requires 500ms. In the absence of a repetitive pattern, on the other hand, it takes1540 ms to move the stage to the image pick-up field of view of alow-magnification reference image at the reference image position. Then,it takes 1 second to move the field of view to the defect position bymoving the stage while at the same time extracting a defect. Finally,the image of the defect position is picked up at high magnification. Thetotal time required is 4.1 seconds. With the additional time required tomove the stage to such an extent that the inspection position is firstcovered by the image pick-up field of view, the total time adds up toabout 5.1 seconds. In the case where a repetitive pattern is found to beinvolved, on the other hand, the image at the reference position is notneeded and therefore the total time is reduced from 5.1 seconds to 3.4seconds.

In the mode 202 shown in FIG. 2B in which the automatic determination ofa repetitive pattern is not carried out, in contrast, 540 ms is requiredfor picking up an image of the reference position, one second for movingthe stage, 540 ms for picking up an image at low magnification, 800 msfor extracting a defect, and 540 ms for picking up an image of a defectposition at high magnification, for a total of 3.4 seconds. This adds upto 4.4 seconds when including the time first required for moving thefield of view to the reference position. It is thus understood that themethod 201 is about one second speedier in the presence of a repetitivepattern, and about one second slower in the absence of a repetitivepattern. All these facts indicate that the throughput is reduced unlessa repetitive pattern appears with the probability of not lower than 50%.

In the sequential processes 203 and 204 shown in FIGS. 2C and 2D,respectively, according to the invention, an image at the inspectionposition is picked up in a wide field of view with a high resolution,thereby eliminating the sequence for picking up an image after defectextraction for a high-magnification image at the inspection position. Inthis way, the increase in the review time can be minimized in the casewhere the image at the inspection position has no repetitive pattern. Asan image having an extracted defect, a high-magnification image pickedup at the inspection position in a wide field of view with a highresolution is displayed on the screen. The image of the defect thusextracted can also be displayed on the screen also after automaticclassification.

In view of the fact that an image of high resolution is picked up in awide field of view, however, a longer image pick-up time of 1800 ms isrequired than in the process 202 for picking up an image at theinspection position. In the case where the image of the referenceposition is not picked up, the processing time is 2800 ms including thetime required for moving the stage to such an extent that the inspectionposition is covered by the field of view. Since the high-magnificationimage is already acquired, the sequential determination of the necessityto pick up an image at the reference position is not required, but canbe carried out concurrently with the subsequent stage movement to theinspection position or the image pick-up process.

In the case where the processing with a single image is impossible andan image is required to be picked up at the reference position, theadditional time is required for moving the field of view to thereference position and picking up an image at the reference position.The performance substantially equal to that of defect extraction fromthe image at the low-magnification inspection position and the referenceposition in the process 201 or 202 can be achieved by processing theimage subjected to the down sampling for reducing the size of the imagepicked up with high resolution to 1/2.5 and extracting a defect. Thedown sampling includes the process of thinning a target image processedthrough a low-pass filter. The down sampling of 1/2.5, therefore, canimprove the S/N ratio with the same effect as in 6.5 sessions of frameintegration.

In the processes 201 and 202, frames are added 12 times while picking upa low-magnification image. The processes 203 and 204, on the other hand,can improve the S/N ratio due to the same effect as if the frameintegration has already been carried out 6.5 times. At the referenceposition, therefore, an equivalent S/N ratio can be achieved once animage is picked up with two frame integrations. In this case, the imagepick-up time is 560 ms including the pre-processing or 1560 ms includingthe stage movement. Thus, the total time is 4.4 seconds. Specifically,it is understood that in the case where the image of the referenceposition is required to be picked up, the processing time can beshortened more than in the process 202 having a higher throughput thanthe process 201. Also, it is understood that in the case where the imageof the reference position is not required to be picked up, a throughputhigher than that of the process 201 which is higher than in the process202. Assume that the frame integration is carried out at the referenceposition the same way as at the inspection position for picking up animage. In the case where an image at the reference position is requiredto be picked up, the time as long as 6.6 seconds is required in all;resulting in the lowest throughput of all.

In the method according to this invention, the high image pick-upmagnification at the defect position is effective up to about threetimes the low magnification. In the case where this limit is exceeded,however, the throughput would be reduced especially when the image atthe reference position is required to be picked up. In the case where itcan be determined before picking up an image at the inspection positionthat the possibility of extracting a defect is high only with the imageat the inspection position, it is desirable that the sequence accordingto the invention including the process 201 or 203 combined with theprocess 204 is employed, while the process 202 is employed in the casewhere the such possibility is low. The possibility of processing onlywith the image at the inspection position beforehand can be determined,for example, based on the method employed when each inspection positionis inspected using another inspection system.

Generally, the semiconductor is inspected by either of two methods. Oneis a chip comparison inspection in which the external appearances at thesame coordinate position of adjoining chips are compared with eachother. The other is a cell comparison inspection in which the patternperiodicity in each chip is utilized. It is known that in the case wherea defect is inspected by the cell comparison method, the inspectionpositions represents a periodic pattern. As a result, a higherthroughput is expected when using the sequence 201 for executing theprocess according to the invention shown. With regard to the chipcomparison method, on the other hand, the process using the sequence 202is expected to produce a higher throughput for a high image pick-upmagnification.

A timing chart according to the invention combined with the process 202is shown in FIG. 3. The periodicity cannot sometimes be determined basedon the method of inspecting each inspection position using anotherinspection device. In such a case, the possibility of defect extractioncan be determined, based on the image of the inspection position alone,by using the design data of the inspection position. Assume that animage of a reference position corresponding to a position different fromthe inspection position involved is picked up as a reference image. Thedesign data for each position is registered generally as a stroke data.Based on this stroke data, the wiring pattern in the field of view atthe reference position is stored as image data.

Next, the wiring pattern in the field of view corresponding to theinspection position involved is converted into image data from thestroke data. The two imaged design data are compared with each otherusing the pattern comparison method, and in the case where the patternsare coincident, it is determined that a defect can be detected from theimage of the inspection position by using the image of the referenceposition corresponding to the different position as a reference image.Also, in extracting a defect using only the periodicity of the patternat the image pick-up position without using the image at a differentinspection position as a reference image, it is similarly possible todetermine using the design data whether a defect can be extracted onlyfrom the image at the inspection position. Specifically, the wiringpattern in the field of view at the inspection position is convertedinto image data from the wiring pattern registered as stroke data, andusing the auto-correlation, it is determined whether the same patternappears at different positions in the field of view at the inspectionposition.

The necessity of picking up an image of the reference positioncorresponding to the inspection field of view involved is not alwaysaccurately determined before picking up the image. As describedpreviously, in many cases where the image appearing in the field of viewcan be predicted based on the design data, the possibility of cellcomparison can be determined before picking up an image of a defect. Inthe actual production line, however, the design data cannot be alwaysacquired. After picking up an image, therefore, it is necessary todetermine, using the image picked up, whether the image at the referenceposition corresponding to the inspection position is required to bepicked up or not.

One method for this determination is consists in determining theperiodicity of the pattern imaged based on the image of the defectposition, for example, and determining that the defect cannot beextracted, using the periodicity of the pattern within the image pickedup at the inspection position in the case where the inspection field ofview is small as compared with the pattern periodicity. In the casewhere the defect size is larger than the pattern periodicity, on theother hand, defects are undesirably compared with each other, andtherefore it becomes difficult to detect the defective area accurately.Especially in the case where the automatic classification or the sizedetermination of a defect becomes necessary, a better performance couldbe expected by determining that the defect cannot be extracted.

The foregoing description of the method of extracting a defect from animage at the defect position presupposes the cell comparison and thecomparison of the reference image. With the logic LSI or the like,however, neither of the aforementioned two methods is applicable to agreater proportion of images at the defect position. The cell comparisonis not applicable in the case where the pattern has no periodicity inthe field of view. The reference image comparison, on the other hand,cannot be used unless the reference image is coincident with the imagein the field of view. This is because the pattern of the logic LSI orthe like is complicated. An effective method of detecting a greaternumber of defects without picking up an image of the reference positionconsists in matching each local area but not the whole of an imagepicked up at the inspection position involved.

First, a method of generating a reference image from the image picked upat the inspection position will be explained with reference to FIG. 4.Numeral 401 designates an image picked up at the inspection positionincluding a wiring pattern. The wiring pattern, though not periodic, isconfigured of wires having substantially the same external appearancehorizontally. Generally, a wiring pattern, which is an artificialobject, has the same external appearance locally, although the defectscan be assumed not to have the same external appearance. In the image atthe inspection position, therefore, the same local area patterns arematched sequentially, and the best match of patterns, e.g. the patternshaving the highest correlationship are compared with each other therebyto generate a differential image. A defect is thus determined based onthe largest image pixel of the differential image, or it is determinedthat an area that has failed in matching is a defect.

Generally, the matching of local areas is accompanied by an increasedamount of arithmetic operation. The use of pyramid matching or the like,however, can reduce the amount of arithmetic operation. Nevertheless,this method harbors the problem of difficulty to apply to the case wheredefects are generated over a wide area. The problem in application ishow to set the local area size.

This point will be explained with reference to FIG. 5. Numeral 501designates a secondary electron image picked up at the inspectionposition, including images of wires 502, 503, 504. Numeral 504designates a defect of which the image picked up is brighter than thewires 502, 503. In the case where a detection system as shown in FIG. 1is used for picking up an image, the wiring may be brightened in thecase where a shorting occurs at any part of the wiring. This is called apotential contrast. In the case where the local area is set to arectangle as indicated by numeral 505, the areas 506 and 505 constituteentirely the same pattern, thereby making it impossible to detect themas a defect.

This problem is solved not by matching the local areas in an imagepicked up at the defect position but by matching them with a local areaof an image determined as a conforming article registered as a referenceimage. In the process, it is more desirable not to presuppose that allthe local areas are rectangles of the same size. This is for reason ofthe fact that in the case where an area to be matched lacks a featuresuch as an edge, accurate evaluation of the matching is difficult. Inthe case where every area is set as a wide are not to cause theaforementioned phenomenon, in contrast, a plurality of wires arecontained in the area, undesirably resulting in the determination thatthe area cannot be matched.

In the area that cannot be matched, the determination of a defect isimpossible. This phenomenon can be effectively prevented by making surethat two or more wiring sets are not included in each local area. In thecase where an arbitrary size of a local area is permitted, however, theamount of arithmetic operation increases vastly. In view of this, thematching is performed with comparatively small rectangular areas, andeach local area is configured by combining such rectangular areas.

An explanation will be given with reference to FIGS. 6A to 6C. In FIG.6A, numeral 601 designates an image picked up at the inspectionposition. In FIG. 6B, numeral 602 designates a reference image alreadypicked up and registered. In the case where the local areas are formedof square areas of the same size as in the image 601, the areascorresponding to those indicated by numeral 603 are absent in thereference image 602, and therefore corresponding local areas formatching cannot be set in the reference image 602. This is attributableto the fact that the local area image 603 includes two wires.

Generally, the thickness and the edge appearance of the wiring in thesame process are imaged in the same manner. The relative positions ofdifferent wires, however, are often different depending on the placewhere the images thereof are picked up. Especially, the two wires imagedin the local area 603 are distant from each other. The minimum distancebetween wires is determined by a particular process. In a semiconductorprocess for which as many wires as possible are required to be formed ina small area with a small distance between the wires, many local areasexist in which matching is possible within a reference image even in thecase where a plurality of wires belong to one local area. In such acase, the image of the inspection position is divided in such a mannerthat wires having a long mutual distance belong to different localareas. The approximate wiring position can be determined by extractingan edge thereof from the image at the inspection position.

The image at the inspection position is divided into comparatively smallsquare areas as indicated by numeral 604 in FIG. 6C, and sampling pointsare set on the edges to secure as constant an interval as possible. Anidentifier of each corresponding square area is determined. Next,considering the sampling points on the edge at an interval wider thanthe previous sampling interval on the edge, and sampling points are setin areas free of an edge in such a manner as to secure as constant aninterval as possible. An identifier of each corresponding square area isdetermined. Each square area corresponding to a sampling point on thedetermined edge or in an area free of the edge is regarded as a nucleusof a different local area. In the subsequent process, each area isexpanded to such a degree that every square area included in the image604 belongs to a local area. The areas which have come into contact witheach other in the process are combined with each other. With regard tothe sampling points on an edge, the expansion speed is increased in adirection in which edges are connected, and decreased in a direction inwhich they are not connected. In this way, a plurality of differentwires can be prevented from belonging to the same local area.

Using this method, a local area having a plurality of different wires isnot easily generated, and there are fewer cases in which a plurality ofimages are required to be matched as a local structure. Thus, thepossibility becomes high of extracting a defect without an image at thereference position. By matching the image of the inspection positionwith a plurality of reference images, on the other hand, the accuracy ofextraction without the image at the reference position is improved.Further, the image pattern of a local area corresponding to the image atan inspection position as well as a reference image can be effectivelycompared with a local area image at other positions in the image at theparticular inspection position. In the case where a local area of theimage at the inspection position is compared with a local area of thesame image, a defect may be overlooked due to the potential contrastdescribed above as an example. This problem can be avoided, however, byfirst comparing the local areas of the same defect position, and nodefect being found there, by comparing the local area image determinedfrom the reference image.

The method described above, however, harbors the problem that thereliability of defect extraction is low for the local area wherematching fails and that a defect expanding to a large area may beoverlooked. Assume that an image at the inspection position having anexternal appearance as indicated by numeral 701 in FIG. 7 has beenpicked up. No other portion having the same external appearance as theportion 702 is found in the area 701. In the case where a semiconductorpattern identical to the pattern 702 is not common, the reference imagehas a smaller chance of having a portion of the same external appearanceas 702. As a result, the portion 702 is liable to be extracted as adefect in spite of its normalcy.

This problem can be solved by use of design data. The correspondinglocal areas should have the same design data, respectively. Thus, thelocal areas having the same design data are compared with each other.The design data generally indicate only the wiring or the holepositions, and often no data is available for the base wiring or thedetailed shape of the wiring. Therefore, the image within a local areaas well as the design data should be used for a decision. A defect isdetermined in the case where the same image of the local areas is notavailable in spite of the fact that the same design data providesreference data having a sufficient number of samples or the image of theinspection position. In the case where there is no local area having thesame design data as the local area in the image of the inspectionposition, in contrast, a defect should not be determined even if theimage of the same local area is not found. The design data of the wiringis generally configured of stroke data. The pattern matching betweenstroke data requires a smaller amount of calculations than the matchingbetween images. Thus, first, the design data are compared with eachother, and then the images in the local areas determined as identicalare compared with each other.

An example configuration is shown in FIG. 8. Numeral 801 designates animage memory corresponding to the memory 108 in FIG. 1. Numeral 802designates design data accumulation unit for storing the design data forthe field of view at the inspection position detected by the systemshown in FIG. 1. Numeral 803 designates reference data accumulation unitfor storing the design data and the image data of an image used as areference image. The data can be accumulated in the reference dataaccumulation unit 803 before starting the review sequence for a group ofinspection points. Nevertheless, the images picked up at the referenceposition in the review sequence can be sequentially accumulated.

Numeral 804 designates image dividing unit for dividing the imageaccumulated in the image memory 801 into a grid of squares, followed bydividing them into local areas using the method described above. Numeral805 designates design data determining unit. The design datacorresponding to each local area generated by the image dividing unit804 is extracted from the design data accumulation unit 802, and matchedwith the design data of the reference image stored in the reference dataaccumulation unit 803 for each stroke data. Numeral 806 designatespattern matching unit. With regard to the local area image determined bythe image dividing unit 804 and the local area image associated with thereference image accumulated in the reference data accumulation unit 803,which are determined as the same design data previously by the designdata determining unit 805, the pattern matching is carried out betweenthe local area image generated by the image dividing unit 804 and thereference image accumulated in the reference data accumulation unit 803.In the pattern matching unit 806, the image is subjected to the subbandanalysis based on the spatial frequency.

The components of each spatial frequency subjected to subband analysiswhich are smaller than a preset threshold level are set to zero in valueand can be ignored in pattern matching. Generally, the place having awiring requires consideration of the phase of the wiring. There are afew local areas with the design data thereof determined as identical inthe design data determining unit 805, thereby leading to a small amountof arithmetic operation for the pattern matching. With regard to theareas lacking the wiring, on the other hand, the resulting lack oflimitation due to the wiring phase increases the number of local areasof the reference image with identical design data as compared with theareas having the wiring. In view of the fact that the spatial frequencyof the areas lacking the wiring contains a fewer high-frequencycomponents, however, the actual amount of arithmetic operation can beconsiderably reduced by the processing to set a weak subband componentto zero.

The processing in the pattern matching unit 806 makes it possible todetermine a local area of the reference image corresponding to eachlocal area in the image picked up at the inspection position, therebyproducing an image corresponding to the image picked up at the referenceposition. Numeral 807 designates a defect extraction unit for generatinga differential image for an area determined to have an identical patternand determining a pixel having a difference as a defect.

The configuration shown in FIG. 8 is such that the matching is carriedout only for the reference image. The image at the inspection positioncan be processed with the same configuration, however, by consideringalso the image at the inspection position in the process executed by thethe design data determining unit 805 and the pattern matching unit 806.Specifically, the design data determining unit 805 compares the designdata of the image at the inspection position stored in the design dataaccumulation unit 802, so that the local areas with the design data at apredetermined distance or more from each other and having identicaldesign data are identified as corresponding local areas. In the patternmatching unit 806, the images of the corresponding local areasdetermined as identical in the design data determining means 805 aresubjected to pattern matching with each other in the same manner as thereference image thereby to determine the corresponding areas.

In the case where the local areas at different inspection positions arecompared, defects of the same type are compared, sometimes resulting inthe same local area image. Even in the case where the local area imagesand the design data corresponding to the local areas are identical, adefect may be determined. Since the reference image can be determinedfree of defect, on the other hand, the condition is always normal in thecase where both the design data and the local area images are identicalbetween the reference image and the local area image.

As described above, the corresponding local area obtained at theinspection position and the corresponding local area obtained in thereference image are different in reliability. A solution to this problemis a method in which a weighting factor is multiplied and theprobability of normalcy is determined as a conditional probability. Thedesign data is used as such a condition. Let ω be the design data at alocal area, and Ω a mass of local areas having the same design data ω.Also assume that x is the image of the local area associated with themass. The conditional probability to be determined is given as P(x|ω).The probability distribution function of this conditional probability iscalculated taking the probability associated with each normal local areainto consideration.

Generally, the probability distribution function is simply a histogramnormalized in the case where X and ω are both satisfied at the sametime. According to this method, the weight of voting is changed withrespect to the histogram, based on an image from which a particularlocal area is obtained. Assume, for example, that a is the probabilitythat the local area image obtained from the image at the inspectionposition is normal and 1 is the probability that the local area imageobtained from the reference image is normal. The local area obtainedfrom the image at the inspection position is multiplied by a, and thelocal area from the reference image is multiplied by 1 thereby tocalculate a histogram. This histogram is normalized to determine aconditional probability distribution function. This method is used tocalculate the normal probability corresponding to each local area, and adefect is determined for a local area having a low occurrenceprobability value.

In the configuration shown in FIG. 8, the image at a reference positionis generated for each local area based on the data of the referenceimage group accumulated in the reference data accumulation unit 803without picking up an image at the reference position. In an applicationof the defect inspection and the review method according to theinvention to a new wafer, it is necessary to make sure that the defectextraction is normally carried out by this technique. For this purpose,the image generated at the reference position is required to beindicated to the user to prompt him to set a parameter or change thedefect inspection mode and the review mode. The system for carrying outthe inspection and the review is required to have the function ofdisplaying both a defect and a reference image generated, which aredisplayed concurrently or alternately with each other. Also, forutilizing this invention actually on a semiconductor production line, itis important to make sure that the aforementioned image is displayed onat least one of the parameter setting screens for sequence control,imaging control and defect detection to prompt the user to switch themode or change the parameters.

It will thus be understood from the foregoing description that accordingto this invention, the operating speed is increased and the throughputimproved for the system for reviewing semiconductor defects. Also,according to this invention, the image at the reference position is notrequired to be picked up in the case where the image pick-up means is aSEM (Scanning Electron Microscope), thereby leading to the advantagethat the reference position making up a normal portion is prevented frombeing contaminated by the electron beam.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description and all changeswhich come within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

1. A method for reviewing a defect, comprising: a defect image pick-upstep for moving a field of view of a microscope to an inspectionposition on a sample based on information from an inspection tool thathas detected a defect and picking up an image at said inspectionposition; a reference image pick-up determination step for determiningwhether an image of a reference position designed to have the sameappearance as said inspection position is to be picked up or not; areference image pick-up step for moving said field of view of themicroscope to said reference position and picking up an image of saidreference position in accordance with a determination result produced insaid reference image pick-up determination step; and defect areaextraction step for extracting a defect area at said inspection positionfrom at least a selected one of just the image of said inspectionposition or both the image of said inspection position and the imagesaid reference position in accordance with the determination result;wherein said reference image pick-up determination step includes atleast a selected one of an image pick-up pre-start reference imagepick-up determination step for provisionally determining, before saiddefect image pick-up step, whether the image of said reference positionis to be picked up or not or an image pick-up post-start reference imagepick-up determination step for finally determining, after said defectimage pick-up step, whether the image of said reference position is tobe picked up or not.
 2. A method of reviewing a defect according toclaim 1, further comprising: an enlarged defect image pick-up step forpicking up an image of said defect area determined in said defect areaextraction step with a magnification larger than in said defect imagepick-up step, an enlarged reference image pick-up determination step fordetermining whether an enlarged reference image of an area having thesame appearance as the area with the image thereof picked up in saidenlarged defect image pick-up step is to be picked up with the samemagnification as in said enlarged defect image pick-up step, and anenlarged reference image pick-up step for picking up said enlargedreference image based on the result of determination result; whereinsaid enlarged reference image pick-up determination step includes atleast selected one of the image pick-up pre-start enlarged referenceimage pick-up determination step for provisionally determining, beforesaid enlarged defect image pick- up step, whether said enlargedreference image is to be picked up or not, or the image pick-uppost-start enlarged reference image pick-up determination step forfinally determining, after said enlarged defect image pick-up step,whether said enlarged reference image is to be picked up or not.
 3. Amethod of reviewing the defect according to claim 2, wherein a selectedone of said defect image pick-up step, said reference image pick-upstep, said enlarged defect image pick-up step, or said enlargedreference image pick-up step is executed concurrently with a selectedone of said image pick-up post-start reference image pick-updetermination step or said image pick-up post-start enlarged referenceimage pickup determination step for an inspection position differentfrom the inspection position.
 4. A method of reviewing a defectaccording to claim 1, wherein the microscope in said defect imagepick-up step and said reference image pick-up step is an electronmicroscope, wherein the microscope in said defect image pick-up step andsaid reference image pick-up step is an electron microscope, wherein animage is obtained by scanning electrons at least once in the imagepick-up field of view at each of an inspection position and a referenceposition, wherein both a defect search image whose number of frameintegrations is the same as that of said reference image pick-up stepand a defect review image whose number of frame integrations is greaterthan that of said reference image pick-up step are produced in saiddefect image pick-up step, and wherein a defect is extracted in saidreference image pick-up determination step by comparing the defectsearch image picked up in said defect image pick-up step with the imagepicked up in said reference image pick-up step.
 5. A method of reviewingthe defect according to claim 1, wherein said defect area extractionstep further includes: statistical amount map calculation step forcalculating the statistical amount indicating the existence probabilityfor selected one of each of the local areas and the pixels of the imageof said inspection positon based on the image of said inspectionposition and the inspection data registered for extracting a defect areafrom said inspection position, and a defect area calculation step forextracting a defect area by comparing the image of said inspectionposition with selected one of the image of said reference position andsaid statistical amount calculated in said statistical amount mapcalculation step.
 6. A method of reviewing a defect according to claim1, wherein the determination is made based on at least a selected one ofthe inspection method for the inspection system used for inspection ofsaid inspection position before said defect inspection review method,the design data of said inspection position or said reference imagepicked up to extract a defect of an object different from saidinspection position.
 7. A method of reviewing a defect comprising: adefect image pick-up step for moving a field of view of a microscope toan inspection position on a sample based on information from aninspection tool that has detected a defect and picking up an image atsaid inspection position; and a defect area extraction step forextracting a defect area at said inspection position from the image ofsaid inspection position or an image stored in a memory, wherein saiddefect area extraction step furthe includes: a statistical amount mapcalculation step for calculating a statistical amount indicating theexistence probability for selected one of each of the local areas andthe pixels of the image of said inspection position based on the imageof said inspection position or stored reference images for extracting adefect area from said inspection position in such a manner thatstatistical amounts in a local area or pixels are calculated from localarea or pixels in the image of inspection position or stored referenceimages that are determined to have the same pattern each other; and adefect area calculation step for extracting a defect area by comparingthe image of said inspection position with a selected one of the imageof said reference position and said reference image is synthesized insaid referemce image generating step.
 8. A method of reviewing a defectaccording to claim 7 wherein said defect inspection review method makesit possible to display a reference image generated in said referenceimage generating step as ari image to the user, and makes it possiblefor the user to confirm the generation of said reference image.
 9. Amethod of reviewing a defect according to claim 7 wherein said referencelocal area is set in said reference image generating step in such amanner that said patterin in said local area is subjected to subbanddecomposition, and each subband component is obtained by subbanddecomposition of selected one of the image of said inspection positionand said stored reference image.
 10. A method of reviewing a defect,comprising: a defect image pick-up step for moving the field of view ofa microscope to a sample inspection position and picking up images ofsaid inspection position by scanning a focused electron beam; areference image pick-up step for moving said field of view of saidmicroscope to a reference position and picking up an image of saidreference position by scanning and focused electron beam; and a defectara extraction for extracting a defect area of said inspection positionusing said image of said inspection position and said image of saidreference position; wherein said image of said reference position ispicked up in such a manner that the number of times of scanning saidfocused electron beam to a reference position in said reference imaepick-up step is smaller than the number of times of scanning saidfocused electron beam to an inspection position in said defect imagepick-up step.
 11. A method of reviewing a defect according to claim 10,wherein said defect image pick-up produces both a defect search imagewhose number of frame integrations is the same as the fram integrationsin said references image pick-up step and a defective review image whosenumber of frame integrations than the fram integrations in saidreference image pick-up step wherein in the defect area extraction step,the defect area is calculated based on said defect search image and saidimage of said reference position.
 12. A method of reviewing a defectcomprising: a defect image pick-up step for moving a field of view of amicroscope to an inspection position on a sample based on informationfrom an inspection tool that has detected a defect and picking up animage at said inspection position; a reference image pick-updetermination step for determining whether an image of a referenceposition designed to have the appearance as said inspection position isto be picked up or not; a referene image pick-up step for moving saidfield of view of microscope to said reference position and picking up animage of said reference position in accordance with a determinationresult produced in said reference image pick-up determination step; anda defect area extraction step for extracting a defeat area in saidinspection position from at least a selected one of just the image ofsaid inspection position or both the image of said inspection positionand the image said reference position in accordance with thedetermination result, wherein said reference image pick-up determinationstep is executed concurrently with said defect image pick-up step orsaid reference image pick-up step.
 13. An apparatus for reviewing adefect detected by an inspection tool comprising: a stage to hold asample to be inspected, said stage configured for translation in X- andY-direction to position said simple in said X- and Y-directions; ascanning electron microscope to irradiate and scan a converged electronbeam onto a sample that is held in said stage, and to detect electronsemanated from said sample by said electron beam; an image memory tostore of images of said sample; an image processor to obtain said imagesand to process said images to produce a precise image of a defect onsaid sample, wherein an image of said sample is obtained by scanning anarea of said sampl with said electron beam, detecting said electrons,and performing one or more frame integrations of detected electrons; anda controller to control positioning of said stage to perform scanninoperations, wherein a defect detected on said sample by an inspectiontool can be positioned within a viewing field of said scanning electronmicroscope to obtain an image of said sample referred to as a defectimage, wherein a reference area on said sample can be positioned withinthe viewing field of said scanning electron microscope to obtain animage of said sample referred to as a reference image wherein the nunberof frame integration of said defect image is greater than the number offrame integration of said reference image.
 14. The apparatus of claim 13wherein said image processor is operable to process said reference imageto have a quality metric substantially the same as that of an imageobtained using the same number of frame integrations as used with saiddefect image.
 15. An apparatus for reviewing a defect detected by aninspection tool comprising: a stage to hold a sample on which aplurality of pattersn are formed and whcih are to be inspected, saidstage being translatable in X- and Y-directions; scanning electronmicroscope to irradiate and scan a converged electron beam onto a samplethat is held in said stage, and to detect electrons emanated from saidsamply by said electron beam; an image memory to stor an image of saidsample; an image processor to obtain said images and to process saidimages to produce a precise image of a defect on said sample, whereinsaid image of said sample is obtained by scanning an area of said samplewith said electron beam, detecting said electrons, and a controller tocontrol positioning of said stage to perform scanning operations, p1wherein said defect area is calculated from a comparison of at least twolocal areas in said image, wherein said image processor is operable toform said local areas that are expected to have similar patterns. 16.The apparatus of claim 15 wherein said controller is operable to controlsaid scanning electron microscope to produce an enlarged image of saiddefect.
 17. A method for reviewing a defect, comprising: moving a fieldof view of a microscope to an inspection position on sample based oninformation from an inspection tool that has detected a defect; pickingup an image at said inspection position; a reference image pick-updetermination step for determining whether an image or a referenceposition designed to have the same appearance as said inspectionposition is to be picked up or not; a reference image generation stepfor generating a reference image; and a defect area calculation step forextraction a defect area by comparing the image of said inspectionposition with said reference image, wherein said reference image isgenerated according to said reference image pick- up determination step,either by moving said fielf of view of the microscope to said referenceposition and picking up an image of said reference position or from apartial image of said image of said inspection portion.
 18. A method ofreviewing a defect according to the claim 17, wherein said referenceimage pick-up determination step is executed concurrently with saidpicking up an image at said inspection position or reference imagegeneration step.